The invention is concerned with maintaining container integrity when subjected to an internal vacuum or negative pressure generated by the cooling of hot fill flowable material, particularly fruit juices and the like which, upon cooling, contract substantially, and to a degree which may distort the container and adversely affect the hermetic sealing thereof. This problem is particularly significant in composite containers, those containers wherein the body is formed of multiple plies of paper, paperboard, or the like.
Various procedures have been put forth for accommodating internally generated vacuums without affecting the appearance or integrity of the container. A particularly prevalent procedure calls for utilization of a metal end cap provided with a central section capable of inwardly deflecting in response to an internal vacuum. However, in order to provide a cap which will respond in this manner, it has heretofore been necessary to use metal ends of substantially less strength than those of the conventional, and preferred, tinplate or steel end. For example, in instances wherein deflection of the end is desired, resort is frequently had either to aluminum, which is significantly more expensive, and/or extremely thin constructions which give rise to problems with regard to the integrity and strength of the seams. Such problems include difficulties encountered in obtaining an airtight peripheral seam between the end and a composite container body, a reduction in the container-rigidifying capability of the end, and a substantial limitation on the depth to which the cap can be inwardly depressed to accommodate a negative pressure. Examples of such vacuum responsive ends will be noted in the following patents:
W. E. Burns U.S. Pat. No. 1,987,817 issued Jan. 15, 1935 PA0 R. M. Creegan U.S. Pat. No. 3,105,769 issued Oct. 1, 1963 PA0 G. F. Chaplin U.S. Pat. No. 3,160,302 issued Dec. 8, 1964
As an alternative to the use of excessively thin or prohibitively expensive end-forming metals, the patent to Fukuoka, No. 4,286,745, issued Sept. 1, 1981, suggests use of conventional tinplate which, as previously indicated, would not normally be responsive to or capable of optimal inwardly depression in response to negative pressure generated within the container. In appreciation of this, Fukuoka proposes a positive inward urging of the bulged end, by a press or the like, prior to a lowering of the temperature of the contents of the container and without recourse to the negative pressure generated. Such a procedure would have to be provided for immediately subsequent to the filling operation as an additional step by those charged with the filling of the containers. This is contrary to the preferred system whereby the container manufacturer presents the user with containers capable of use in a conventional filling operation and without recourse to additional manipulative steps.